1. Field of the Invention
This invention relates to a galvanizing alloy and process and, more particularly, relates to a galvanizing alloy and an immersion galvanization process adapted to control the undesirable effects associated with galvanizing reactive steels.
2. Description of the Related Art
The conventional process for hot dip galvanizing of low carbon steels comprises pretreatment of said steels in a 20% to 30%, by weight, zinc-ammonium-chloride (ZnNH.sub.4 Cl) pre-flux, followed by immersion in molten zinc or zinc alloy baths. The `normal` or `N` coating structure produced on low reactivity steel by conventional hot dip galvanizing processes has well defined, compact alloy intermetallic layers. The predominant growth mode in this type of coating is by solid-state diffusion of iron and zinc, and thus well established intermetallic delta and zeta layers control the rate of the galvanizing reaction. The diffusion reaction rate decreases as the coating thickness increases, thus permitting predictable, consistent coverage. The normal coating has a bright metallic luster.
Recent developments in the manufacture of low-alloy high-strength steels include continuous casting. In the continuous casting process, it is necessary to add elements that `kill` or deoxidize the steel, i.e., prevent gaseous products that produce porosity. Silicon is commonly employed for this purpose. The resulting steels generally contain between 0.01% to 0.3%, by weight, silicon but may include up to or more than about 0.5 wt % silicon and are known as `reactive steels` or silicon steels.
Phosphorus in the steel also affects reactivity, having an accepted measure of reactivity that is approximately 2.5 times that of silicon. Thus, the silicon content plus 2.5 times the phosphorus content is known as the effective silicon content of the steel.
Silicon steels that have high high reactivity pose problems to the galvanizing process, producing thick, brittle and uneven coatings, poor adherence and/or a dull or marbled appearance. These coatings are known as `reactive` coatings. The high reactivity of the silicon steels also causes excessive zinc consumption and excessive dross formation.
Silicon released from the steel during galvanizing is insoluble in the zeta layer, which creates an instability in that layer and produces thick, porous intermetallic layers. The microstructure is characterized by a very thin and uneven delta layer overlaid by a very thick and porous zeta layer that allows liquid bath metal to react near the steel interface during the entire immersion period. The result is a linear growth mode with immersion time that allows the formation of excessively thick coatings. These undesirably thick coatings are generally very rough, brittle, and dull in appearance.
Steels with silicon levels between 0.05 to 0.15 (i.e. around the "Sandelin Peak" area). may also develop a `mixed` reactivity or `M` coating, which is characterized by a combination of reactive and non-reactive areas on the same steel that is believed to be the result of differences in localized silicon levels on the surface of the steel.
It is known in the prior art to control reactivity by producing bath temperature and immersion time at a rate inversely proportional to the silicon content of the steel. Lower bath temperatures, on the order of 430.degree. C., and reduced immersion times tend to control the reactivity of high silicon steels. However, using low bath temperatures and reduced times on low silicon steels produces unacceptably thin coating thicknesses. Thus, the galvanizer must know the silicon content of the steel beforehand and adjust the hot dip parameters accordingly. This approach cannot be implemented if steel reactivity is not known or if components to be galvanized comprise parts of different reactivities welded together. With low-temperature galvanizing, productivity can be poor because of the need to increase immersion times.
It is also known to control steel reactivity by adding alloy elements to the zinc galvanizing bath. One such addition is nickel in a process known as the Technigalva.TM. (or Nickel-Zinc) process A nickel content of 0.05 to 0.10% by weight in the zinc bath effectively controls reactive steels having up to about 0.2% by weight silicon content. For steels having silicon levels above approximately 0.2 wt. %, this nickel-zinc process is not effective and thus it is only a partial solution to the reactive steel galvanizing problem. Normal steels of low reactivity, when galvanized by the nickel-zinc process, pose the same difficulty as seen in low temperature galvanizing in that coating thickness may be unacceptably thin. With this process, it is thus preferred that the galvanizer know the reactivity of the steel beforehand and adjust galvanizing conditions accordingly, both of which are difficult to accomplish in practice. Under some conditions, this process also produces dross that tends to float in the bath and be drawn out on the workpiece, producing unacceptable coatings.
Another alloy used to control reactivity is that disclosed in French Patent No. 2,366,376, granted Oct. 27, 1980, for galvanizing reactive steels, known as the Polygalva.TM. process. The alloy comprises zinc of commercial purity containing, by weight, 0.1 to 1.5% lead, 0.01 to 0.05% aluminum, 0.03 to 2.0% tin, and 0.001 to 2.0% magnesium.
U.S. Pat. No. 4,439,397, granted Mar. 27, 1994, discusses the accelerated rate at which the magnesium and aluminum are consumed or lost in this Polygalva.TM. process for galvanizing steel. Procedures are presented to overcome the inherent difficulty in replenishing deficient aluminum or magnesium in the zinc alloy galvanizing bath. The process has serious limitations in that the steel has to be meticulously degreased, pickled, pre-fluxed, and oven-dried to obtain good quality product free of bare spots. Thus, in most cases, new high-quality installations are usually required.
U.S. Pat. No. 4,168,972, issued Sep. 25, 1979, and U.S. Pat. No. 4,238,532, issued Dec. 9, 1980, also disclose alloys for galvanizing reactive steels. The alloys presented include variations of the Polygalva.TM. alloy components of lead, aluminum, magnesium, and tin in zinc.
It is known in the prior art that aluminum included in the galvanizing bath reduces the reactivity of the high silicon steels. A process known as the Supergalva.TM. process includes an alloy of zinc containing 5 wt. % aluminum and requires a special flux and double dipping not generally accepted by commercial galvanizers.
Co-pending U.S. patent application Ser. No. 08/667,830 filed Jun. 20, 1996 now abandoned, the disclosure of which is incorporated herein by reference, describes a new alloy and process for controlling reactivity in steels with silicon content up to 1 wt. %. The alloy comprises zinc of commercial purity containing, by weight, one or both of vanadium in the amounts of at least 0.02% to 0.04% and titanium in the amounts of at least 0.02% to 0.05%.
It is a principal object of the present invention to provide a process and alloy to effectively control reactivity on a full range of steels, including low and high silicon steels. The process should also produce coatings of acceptable and uniform thickness over the full range of steels.
Another object of the invention is to provide an alloy and process that uses standard galvanizing equipment operated under normal conditions for galvanizing steels of mixed reactivity without the need to adjust for variations in steel chemistry.